1. Field of the Invention
The present invention relates to a power converting apparatus capable of controlling an output current. More particularly, the invention pertains to an active filter for suppressing harmonic current components which may leak out of a load apparatus and flow into a power supply by injecting a harmonic compensation current for canceling the harmonic current components.
2. Description of the Background Art
An active filter is an apparatus which generates a harmonic compensation current for suppressing harmonic currents produced by a load apparatus. Connected to a power supply line of the load apparatus at a point close thereto, the active filter injects the harmonic compensation current into the power supply line to cancel out the harmonic currents leaking out of the load apparatus. A conventional active filter detects a load current, separates the load current into a fundamental component and harmonic components thereof, extracts only the harmonic current components of the load current, and generates a harmonic compensation current which is offset by 180 degrees in phase from the harmonic current components. The harmonic compensation current generated by the active filter is injected into the power supply line from a load connection point to cancel the harmonic currents contained in the load current, whereby a source current is reshaped into a sine wave containing no harmonics. This kind of active filter typically employs as a current source a high-frequency pulse width modulation (PWM) inverter which is characterized by low loss, as described in a non-patent publication entitled “Introduction to Power Electronics, Second Edition,” Sakae Yamamura and Eiichi Ohno, pp. 264-269, published by Ohmsha, Ltd.
As the conventional active filter employs the high-frequency PWM inverter as mentioned above, the active filter has needed a large-scale filter circuit for suppressing leakage of harmonic currents to power supply side, making it difficult to manufacture an active filter system in a compact structure.